Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches

[Nick Desaulniers]

On Tue, Aug 18, 2020 at 12:19 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, Aug 18, 2020 at 12:03 PM H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> >
> > I'm not saying "change the semantics", nor am I saying that playing
> > whack-a-mole *for a limited time* is unreasonable. But I would like to go back
> > to the compiler authors and get them to implement such a #pragma: "this
> > freestanding implementation *does* support *this specific library function*,
> > and you are free to call it."
>
> I'd much rather just see the library functions as builtins that always
> do the right thing (with the fallback being "just call the standard
> function").
>
> IOW, there's nothing wrong with -ffreestanding if you then also have
> __builtin_memcpy() etc, and they do the sane compiler optimizations
> for memcpy().
>
> What we want to avoid is the compiler making *assumptions* based on
> standard names, because we may implement some of those things
> differently.
>
> And honestly, a compiler that uses 'bcmp' is just broken. WTH? It's
> the year 2020, we don't use bcmp. It's that simple. Fix your damn
> broken compiler and use memcmp. The argument that memcmp is more
> expensive than bcmp is garbage legacy thinking from four decades ago.
>
> It's likely the other way around, where people have actually spent
> time on memcmp, but not on bcmp.
>
> If somebody really *wants* to use bcmp, give them the "Get off my
> lawn" flag, and leave them alone. But never ever should "use bcmp" be
> any kind of default behavior. That's some batshit crazy stuff.
>
>                Linus

You'll have to ask Clement about that.  I'm not sure I ever saw the
"faster bcmp than memcmp" implementation, but I was told "it exists"
when I asked for a revert when all of our kernel builds went red.
-- 
Thanks,
~Nick Desaulniers